Question

Sa se calculeze cos 105^ + cos 75^.

Answer 1

Răspuns:

0.

Explicație:

cos a + cos b = 2 cos (a+b)/2 * cos (a-b)/2

cos 105° + cos 75° = 2 cos (180°/2) * cos (30°/2) =

2 cos 90°  *  cos 15° =

2 * 0 * cos 15° =

0.

Answer 2

$\it cos75^o = -cos(180^o-75^o)=-cos105^o\ \ \ (*)\\ \\ cos105^o+cos75^0\ \stackrel{(*)}{=}\ cos105^o-cos105^o=0$